Metal cap and its method of manufacture

ABSTRACT

A metal cap having a primer layer thereon which has a thermal adhesiveness with respect to a lining layer and wherein part of the lining at the bottom of a cap is thermally adhesively fixed to the primer layer. 
     A method of manufacturing a metal cap including steps of applying a primer to the cap which has a thermal adhesiveness with respect to a lining layer and heating only a portion of the bottom of the cap to cause only part of the lining to become affixed to the primer.

This is a division, of application Ser. No. 39,119, filed May 15, 1979,now U.S. Pat. No. 4,272,313.

TECHNICAL FIELD

The invention relates to a metal cap and a method for its manufacturewhere the cap is adapted to seal the open ends of containers and wherethe cap has a lining layer thermally adhesively affixed to a primerlayer contained on the cap over a portion of the inside bottom surfaceof the cap.

BACKGROUND ART

Metal caps are used for sealing the open ends of containers such asbottles or wide neck jars and comprise a metal shell to which a lininglayer has been added. The metal shells are made by shallow stamping ordeep stamping of metal sheets with threading or pleated flare processingadded as required. Specifically metal caps include items such as crowns,screw caps, pilfer-proof caps and twist-off caps. The metal shellportion of the caps may be uncoated, may be treated with ananti-corrosion coating, and may have printing applied. Shell materialssuited for use in the present invention include both non-magnetic metalmaterials such as aluminum, aluminum alloy, copper and brass andmagnetic metal materials such as tin plate, tin free steel and coatedsteel plate.

In order to prevent the seal lining layer used in a cap from separatingfrom the cap during transport or during supply to cap supply chutes inpacking and sealing apparatus, the lining layer is ordinarily adhered tothe entire inner surface of the bottom of the metal shell by means of aprimer layer. However, with the recent appearance of caps, andparticularly crowns, used as prizes, there have been various proposalsfor metal caps in which the lining layers are easily peelable from themetal shells. Prize marketing caps include those having a winning ticketfor a prize or other characters, codes or drawings printed as marks onthe inside bottom surfaces of metal caps with the idea of promoting themarketing of the bottles. In order to make the peeling of the lininglayer easy in this type of prize cap, only the center part of the lininglayer is lightly affixed to the metal cap leaving all or part of theouter periphery of the lining layer in a non-adhesive state. Forexample, the method of manufacture of some caps includes the step ofhaving adhesive primer painted only in the center part of the bottominside surface of the metal shells, while other methods included havingthe entire bottom inside surface of the shells painted with adhesiveprimer, and then having ink that will prevent adhesion between thelining layer and adhesive primer painted in part or all of the outerperiphery of the bottom surfaces. However, both of these methods requirespecial types of primer painting apparatus adding a step to themanufacturing process and both methods require precise control of thepainting process to prevent poor registration.

A problem which exists where a lining layer comprising a resin such aspolyethylene is used, is that the lining layer, if it is adhered overthe entire bottom surface of a cap, will tend to crack along its outerperiphery over a period of time after bottling leading to a loss ofsealability. This is in part due to stress cracking characteristics ofpolyethylene material.

It is therefore an object of the present invention to provide for ametal cap and method of manufacture of the same which does not requireany complicated adhesive primer painting process to affix a part of alining layer to a primer layer over a portion of the inside bottomsurface of a metal cap.

DISCLOSURE OF INVENTION

Broadly a metal cap constructed according to the invention has a lininglayer overlying a primer layer on the inside bottom surface of the cap.The primer layer has a thermal adhesiveness with respect to the lininglayer such that when a portion of the bottom surface of the cap isheated, only that part of the lining layer at the heated portion willbecome affixed to the primer.

The method of manufacture of the cap described above includes applyingheat to only the bottom portion of a metal cap where it is desired tohave a lining material affixed to a primer. The heating of the bottom ofthe cap is preferably accomplished by inducing a high frequencyelectrical current in the portion of the cap where it is desired to havethe lining layer affixed to the primer layer. The heating step can bedone before application of the lining material in which event the liningmaterial is cut from a sheet to a desired shape and pressed onto theheated portion of the bottom of the cap.

In a further embodiment, the lining material may be applied prior toheating in which event the material is pressed after heating by a cooledpunch to shape the material into a sheet form.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side sectional view of a metal cap constructed according tothe invention;

FIG. 2 is an enlarged view of the bottom portions of the cap of FIG. 1;

FIG. 3 is a diagrammatic plan view of the adhesive and non-adhesiveareas of the lining layer available in a cap constructed according toFIG. 1;

FIG. 4 is a partial plan view of an apparatus for manufacturing the capillustrated in FIG. 1;

FIG. 5 is an enlarged sectional view of FIG. 4 taken along lines A--A;

FIG. 6 is a diagrammatic view illustrating the flow of induced currentin a metal shell;

FIG. 7 is a graph illustrating the relation between heating time andtemperature of a bottom of a metal shell; and

FIG. 8 is a graph illustrating temperature variation in a bottom of ametal shell measured from the center of the cap.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, there is illustrated a metal cap 1 having a metalshell 2 which forms the main body of the metal cap. The shell has anadhesive primer layer 3 thereon which may directly contact the shell orindirectly through means of an anti-corrosion coating interposed betweenthe shell and the primer layer. As shown, a lining layer 4 is affixed tothe primer layer 3 through a thermal adhesive portion 6 and contacts theprimer layer through non-adhesive portions 5.

FIG. 2 illustrates in greater detail the positioning of the layers andin addition the positioning of anti-corrosion coatings 2' and 2" on thesurface of the shell 2.

Referring to FIGS. 3a-3d, there is illustrated the positioning of highfrequency conductors 17 which are utilized to heat portions of thebottom of a shell in a manner described hereafter and which in additionillustrates various configurations of non-adhesive parts 5 and adhesiveparts 6 that may be obtained by different positioning of the conductors17. FIG. 3a illustrates adhesive part 6 extending in two parallel stripsto divide the lining layer into three nearly equal parts with the widthsof adhesive part 6 being either nearly equivalent to the outer diametersof the high frequency current conductors 17, or slightly larger.Non-adhesive parts 5 occupy the outside and inside of adhesive parts 6and consequently non-adhesive parts 5 occupy the greater part of theinside and the greater part of the outer periphery of the lining layer.The outer periphery in this instance is taken to mean the part having anarea inside and along the outer perimeter.

FIG. 3b shows adhesive part 6 extending through the area of the centerof the lining layer and to make a near rectangular shape having a widtha little larger than two times the outer diameter of the high frquencyelectric conductor 17. Here non-adhesive part 5 occupies the greaterpart of the outer periphery of the lining layer.

FIG. 3c shows adhesive part 6 formed in a nearly circular shape in thecenter of the lining layer, and as will be discussed later, this is theshape used when heating the shell while rotating it around its centerover two adjoining high frequency current conductors 17 whose currentdirections are mutually opposite. In this case non-adhesive layer 5occupies the entire portion of the outer periphery of the lining layer.

FIG. 3d is an example of the adhesive part 6 being formed to make a ringon the peripheral part of the lining layer. This is accomplished bypositioning the conductors under the periphery of the shell bottom, andheating while rotating the shell.

The lining material used may include any thermoplastic resin (rubberincluded) having elastic properties suitable for use as a sealingmaterial, for example soft vinyl chloride, styrene-butadiene-styrenecopolymer, linear polyamide resin, fluorine resin and polyolfin resin.Since polyolefin resin such as polyethylene has superior sanitary,anti-moisture and mechanical properties, it is well suitable as liningmaterial. Polyolefin resins that may be used include polyolfins such aslow density, medium density and high density polyethylene,polypropylene, polybutene-1, ethylene-butene-1 copolymer andethylene-propylene copolymer, ethylene-vinyl acetate copolymer, andmodified polyolefins or olefin copolymers containing as their mainingredients olefins such as unsaturated carboxylic acid modifiedpolyethylene or polypropylene, and considerable amounts of ethyleneunsaturated monomer other than olefin. These olefin resins can be usedsingly or in combination of two or more, and they can also be blenedwith elastomers such as ethylene-propylene rubber and butyl rubber toimprove the elastic properties needed for sealing materials. It is alsopossible to blend oxidation inhibitors, heat stabilizers, lubricants,fillers or colorants into these polyolefins. Further it is also possibleto blend cross-linking agents and foaming agents either singly or incombination to obtain polyolefin resin of superior mechanical propertiessuch as elasticity.

These lining layers may be in the form of flat sheets, or as shown inFIG. 1, they may be furnished with a peripheral projection that contactsthe upper end of the container mouth and this projection may assume anannular shape that clasps both sides of the mouth as required.

While it is important that the primer material have a thermaladhesiveness to the lining material comprising a thermoplastic resin, itis also important that the primer material has adhesiveness to the metalshell itself or to the anti-corrosion coating 2' formed on its innersurface. This is important in order to prevent the lining layer fromseparating from the metal shell during transport or during supply to thecap supply chute of the packing and sealing apparatus and also toprevent leakage of the liquid contents as described later.

Examples of such primer materials are adhesive resins such asethylene-vinyl acetate copolymer, ethylene-acrylic acid ester copolymer,epoxy, polyurethane, polybutadiene, epoxyurethane, modified polyolefin,ionomer or resins in which they are the main ingredient or theirmixtures. Primers particularly well suitable for use in the presentinvention are resins having polyolefin oxide or modified polyolefindispersed in the base resin used to form the primer coating. Examples ofbase resins are epoxy resin, phenol resin, amino resin, polyester resin,alkyd resin and heat curing alkyd resin monomer or combinations of twoor more of these or other thermoplastic resins.

Polyolefin oxide is a substance obtained by oxidizing polyolefins suchas polyethylene or polypropylene or their copolymers in the molten ordissolved state. The oxygen therein is believed to be present partly inthe form of polymer chain end carboxyl groups or carboxylic estergroups, and partly in the form of either groups and ketone groupsintermediate on the polymer chain. Modified polyolefin means polyolefingraft modified mainly by unsaturated carboxylic acid or its derivates(at a graft rate preferably of 0.001-10 wt %).

The above primer will ordinarily be obtained by mixing a solution of thebase resin dissolved in an organic solvent with a solution of polyolefinoxide or modified polyolefin dissolved in such as hot xylene or Decalin.Ordinarily the polyolefin oxide or modified polyolefin will be 3-30parts by weight per 100 parts by weight of the base resin.

The paint amount (nonvolatile fraction) of the primer layer should beabout 30-150 mg/dm². The primer layer may cover the entire inner surfaceof the metal shell or it may cover only the bottom part.

In this case, the polyolefin oxide or modified polyolefin in the primerlayer will ordinarily have a melting point or softening point of atemperature capable of thermal adhesion with the lining layer.

Anti-corrosion coating 2" is not necessarily required in cases when theprimer layer covers the entire inner surface of the metal shell, butwhen it is applied, it will be selected from known undercoating paintssuch as phenol-epoxy, epoxy-amino and phenol-epoxy-vinyl resins.

In the case of prize caps, the prize marking is printed onanti-corrosion coating 2" or on the primer layer.

The metal cap of the present invention is manufactured as follows.

An anti-corrosion coating and/or printing 2' is applied to one surfaceof a metal sheet that is to comprise the shell material, preferablyaluminum or aluminum alloy sheet. An anti-corrosion coating 2" isapplied to the other surface of the sheet if such coating or printing isdesired. A primer is then applied onto anti-corrosion coating 2" afterwhich the sheet is annealed. The annealed coated sheet is then pressedinto cap form with the primer layer being on the inner surface.

It is important that the bottom inner surface of the metal shell thathas been formed be heated in a manner that a part of the lining layer isthermally adhered. There are no particular restrictions as to the methodof heating the shell bottom for this purpose and any heating method canbe adopted such as local hot blast heating, frame heating, infrared rayheating, electric resistance heating, laser heating, electron beamheating and high frequency induction heating. However methods other thanhigh frequency induction heating will have difficulty in sufficientlysatisfying the conditions needed for the industrial production of themetal cap of the present invention such as heat efficiency, heatingatmosphere, compactness of apparatus, high speed continuous heating,temperature control and local heating only in desired positions on theshell bottom.

Consequently it is preferred that the high frequency induction heatingmethod be adopted in the manufacture of the metal cap of the presentinvention.

Referring to FIG. 4, there is illustrated a continuous high frequencyinduction heating apparatus 11 by which a metal shell may be heated. Aplurality of semicircular notches 13 are provided at intervals in theperipheral part of a rotating table 12 comprising a transport means forshells 15 and shells 15 are charged to notches 13 from a chute 14 withtheir bottom sides down. A charged shell 15 is transported in therotational direction of the rotating table 12 (direction of the arrow inthe drawing) and is charged to a heating apparatus 11. The shell slideson guides (not illustrated) positioned before and after the heatingapparatus.

Heating apparatus 11 comprises high frequency electric conductors 17connected to a high frequency power source 16. A guide panel 18 andvertical guide 19 also form part of the heating apparatus.

FIG. 5 shows a vertical section of the heating apparatus 11 and in thatfigure a pair of high frequency electric conductors 17 are illustratedwhere the direction of the current in each conductor is mutuallyopposite.

As shown in FIG. 6, when the directions of the current in the conductorsare mutually opposite, the induction current that is excited in thebottom of the shell directly over the conductors and the shell wallforms a closed circuit 22 in which the current flows in large amounts sothat a local temperature rise is efficiently accomplished by Jouleheating. If the directions of the current were the same, the closedcircuit shown in FIG. 6 would not be formed and the efficiency ofinduction current excitation would be low resulting in littletemperature rise. Further no temperature rise would occur in the case ofa single conductor as no circuit would be formed.

Ordinarily when a metal shell comprises a magnetic material such as tinplate, induction heating occurs at good efficiency because the magneticflux concentrates on the bottom of the shell. But in the case of anon-magnetic shell, such as aluminum, efficiency is poor because ofconsiderable leakage of the magnetic flux. Yet in either case, inductioncurrent flows only in the parts directly over the conductors or in theirvicinity. Consequently, since the temperature of the shell areas outsidethe heated portions directly over or nearly over the conductors risesonly because of heat conduction from the heated portions, these outsideareas will remain at lower temperatures during the initial heatingphase. However with passage of time, the temperature rise over theconductors slows down and since the metal is a good heat conductor, thetemperature difference between the two parts decreases. Consequently,thermal adhesion of just a part of the lining layer is difficult. Thetime involved will vary depending on the dimensions of the shell, sheetthickness, material, distance between the conductors and the shellbottom, diameter of the conductors, distance between the conductors,strength of the high frequency current and its frequency. Ordinarily asshown in FIG. 7, the shell heating time, that is the time for passagethrough heating apparatus 11, should be about one second maximum, abouttwo seconds maximum and about four seconds maximum respectively forshell bottom diameters of 15-25 mm, 26-40 mm and 41-60 mm.

FIG. 7 using the thermal adhesion configuration illustrated in FIG. 3c,shows the relation between: (a) shell heating time, and (b) thetemperature in the central part of the shell when the shell has beenheated by high frequency induction (line 1a and 2a) and the temperatureof a point outside the center of the shell by a distance of radius ×2/3(curves 1b and 2b). The measurements were obtained by thermopaint. Inthis Figure, lines 1a and 1b show a case when a tin plate shell 27 mm indiameter and 0.27 mm thick is heated by the heating apparatus 11 of FIG.4 using a 400 kHz high frequency power source and high frequency heatingcoils with the distance between the centers of the conductors being setat 7 mm, conductor diameter 4 mm, conductor length, (current flow in oneconductor) 800 mm, and distance between the conductor ends and the shellbottom at 0.3 mm. FIG. 7 shows an example of temperature variations atthe shell center and at a point separated from the center by radius×2/3. Further, the output was regulated so that the temperature in thecenter part was a constant 160° C.

Also, lines 2a and 2b of FIG. 7 respectively show the case when analuminum shell 38 mm in diameter and 0.2 mm thick was heated by heatingapparatus comprising a distance on centers between conductors of 10 mm,conductor diameter 6 mm, conductor length 1,000 mm, and a distancebetween the conductor ends and the shell bottom of 0.3 mm, and show anexample of temperature variations at the shell center and at a pointseparated from the center by radius ×2/3.

The interval between conductors can be designed for any desired positionor size of non-adhesive parts, for example, they can be separated as inFIG. 3a or can be adjoining as in FIG. 3b. The diameters of theconductors will generally increase together with the diameter of theshell. For example, for shell diameters of 15-25 mm, 26-40 mm and 41-60mm, the conductor diameters should respectively be about 3 mm, about 5mm and about 7 mm. This is because the greater the shell diameter, thegreater the need for larger current flows in the conductors and when thediameter of the conductor is small, the Joule loss in the conductorbecomes large. On the other hand, when the conductor diameter increases,the electromagnetic coupling between the conductor and the cap decreasesand heating efficiency declines. Further it is desirable to have thediameter of the conductor as small as possible in order to impart anon-uniform temperature distribution.

In order to prevent occurrence of variations in induction current amounton the shell bottom because of slight differences in the distancebetween the shell bottom and the conductors and the consequentvariations in heating temperature and in order to prevent short circuitsbetween the shell bottom and the conductors, the conductors should becontained, as shown in FIG. 5, in attachment 20, which is made ofsynthetic resin such as Bakelite (when using vacuum tube-typegenerators) or of an insulating high permeability material such asferrite (when using transistor-type generators). An adhesive 21 such asepoxy resin holds the conductors in the attachment so that theconductors will not move unstably during heating.

High frequency power source 16 can be any type desired but a vacuumtube-type should be used for high frequencies of 100 kHz to 10 MHz, anda transistor-type should be used for frequencies on the order of 10 kHzto 80 kHz.

The smaller the distance between the conductors and the shell bottom,the larger the non-uniformity in heating temperature on the shell bottomand the greater the rise in heating efficiency. Consequently when highlypermeable material is not used in attachment 20, the distance betweenthe upper ends of the conductors 17a and shell bottom surface 15b shouldbe 1 mm maximum and preferably 0.5 mm maximum. Distances closer thanabout 0.1 mm may lead to short circuits and should not be used. Whenusing insulating high permeability materials in the attachment, thedistance between attachment upper surface 20a and shell bottom surface15b should be 2 mm maximum, and preferably 1 mm maximum. In this case,since the ferrite ordinarily used as the insulating high permeabilitymaterial is brittle, it is necessary to protect the upper surface of theattachment with an insulating sheet such as Bakelite of a thickness ofabout 0.4 mm, so that it will be difficult to make the distance betweenthe upper surface of the attachment and the shell bottom lower than 0.4mm.

In the case of a shell that is non-magnetic such as aluminum, the abovedistances are regulated by varying the height of guide panel 18. Hereguide panel 18 receives and supports the shell after it floats up fromthe repulsion force formed by the high frequency coil magnetic field andthe induction current induced in the shell and its lower surface 18a hasthe function of sliding along open end 15a of shell 15 as it is drivenby transport means 12.

The best guide panel is a strengthened glass plate with a smoothsurface. When the shell is made of magnetic material such as tin plate,it will not ordinarily float up so that the distance is regulated by thethickness of an insulating film such as Teflon film placed over theconductor.

When it is desired to form a non-adhesive part as shown in FIG. 3c byrotating the shell during heating, a lining of a low friction material,such as Teflon is applied, in the parts where the shell contacts notch13 while a lining of an elastic matrial such as silicon rubber having acomparatively higher friction coefficient is applied to the innersurface of vertical guide 19 with which the shell comes in contact. Theshell will then be rotated when moved by the transport means 12transported with its walls pressing against the inner surface ofvertical guide 19.

A shell that has been heated as described above then passes throughguides, not illustrated, and enters molten thermoplastic resin supplyapparatus 24 where an extrusion apparatus supplies a moltenthermoplastic resin particle nearly in the center of the shell's innersurface.

Then the cap is delivered to a press station, not illustrated, where themolten thermoplastic resin particle is compressed and elongated to sheetform with a cooled punch and hardens to become a lining layer thermallyadhered to those parts of the shell bottom heated to those temperatureswhere thermal adhesion is possible.

The present invention will not be restricted to the example describedabove, for instance, it is possible to drop the particle of moltenthermoplastic resin into the shell before heating, or, after inserting athermoplastic resin film cut to the prescribed form to heat the shelland then adhere by pressing.

The metal cap of the present invention is one where the lining layer canbe easily peeled from the primer layer making it well suitable for useas a prize cap. Peelability of the lining layer can be easily regulatedby such as heating temperature and by selection of combinations of linerresins and the resins that will comprise the primer layer. For example,when a polyethylene liner is adhered to a primer layer comprising a baseresin including polyethylene oxide and modified polyethylene, it ispossible to make peeling easy by using modified polyethylene orpolyethylene oxide modified to a low degree, or to lower the adhesivestrength by reducing the content of both.

Weak adhesion begins from the point where the heating temperature of theshell bottom is about 30-40° C. lower than the softening point of thepolyolefin oxide or modified polyolefin, and up to a certain fixedtemperature, the thermal adhesive strength increases as the temperaturerises. Since the highest temperature on the shell bottom is at the partdirectly over end 17a on the conductors and the temperature graduallydecreases away from this part, a slope in adhesive strength (peelingstrength) occurs in the thermal adhesion part, as shown for the exampleof the present invention (A-1) in Table 1. This is a great point ofdifference with prior metal caps where the adhesive primer was paintedlocally or where an adhesion inhibiting ink was printed to make thelining layer locally adhesive. The metal cap of the present invention,particularly the cap where the vicinity of the center of the shellbottom is made thermally adhesive as shown in FIG. 3c has thecharacterizing feature, as described above, that the adhesive strengthgradually increases toward the center of the bottom. Consequently,because of the residual stress in the radial direction formed in theliner when the liner is formed by press forming the particle of moltenthermoplastic resin (for example, polyethylene), there is no likelihoodthat the liner will peel over passage of time. Also the outer peripheryof the lining layer, that is part 4a presses against the container mouthupper part during capping, is in a nonadhesive state with the primerlayer and is thus easily modified under pressure imparted during cappingso that compared to prior metal caps comprising adhesive lining layersover their entire surfaces, no unreasonable stresses impinge on the saidpart 4a. Consequently, it has the characterizing features thatenvironmental stress destruction through the medium of the containedbeverage will have difficulty occuring, and leakage of the contentsbecause of incomplete sealing will also have difficulty occurring.

The invention is further explained by reference to the following.

EXAMPLE 1

A primer was made by dissolving or blending 90 parts by weight of epoxyresin, 10 parts by weight of phenol resin, and 7 parts by weight ofpolyethylene oxide (mean molecular weight 6,500, density 0.98, acidvalue 13.0, softening temperature 122° C.) in a mixed organic solvent(equivalent solvent mixture of methylisobutyl and methylethylketone) sothat the solid fraction comes to 30 wt. %.

The paint was painted onto degreased lustrous 0.27 mm thick tin platewith the surface tin melted and heating was done in an electric oven at200° C. for 10 minutes to obtain a coated tin plate (A) having a coatingof 100 mg/dm².

Then, a part of coated tin plate (A) was printed with an adhesioninhibiting alkyd ink in an annular shape 26.6 mm outer diameter and 16.6mm inner diameter to make partially printed coated tin plate (B).

The above coated tin plates (A) and (B) with their coated surfacesinside were made into crown seals (diameter 27 mm) by a conventionalpress forming method. However, with coated tin plate (B), the annularpartial printing was formed so that it registered exactly at the outerperiphery of the crown seal bottom.

From the above two types of crown shells, the following three types weremade comprising one type being a crown following the present inventionand two other types following prior technology to serve as comparisons.

(1) Crown A-1 following the present invention.

Crown shells made from coated tin plate (A) were heated with highfrequency in the manner shown in FIG. 3c, with the heating apparatus ofFIG. 4. At this time the high frequency induction heating coil having acoil conductor distance on centers of 7 mm, conductor diameter of 4 mmand conductor length of 800 mm was arranged so that the distance betweenthe upper end of the coil conductor and the shell bottom was 0.3 mm. Thetemperature distribution on the shell bottom was as shown by curve 1 inFIG. 8 from the results of measurements of the shell bottom surfacetemperature by thermopaint with an input of 15 kW to a 400 kHz highfrequency heating apparatus and with the passage of the shell throughthe heating coil in 0.7 seconds. Also, the shell made about ninerevolutions during this time. A 900 mg particle of molten polyethylene(MI=3, density 0.918) was inserted into the crown shell which had beenextruded from an extruder (diameter 40 mm, L/D 16 ) fitted with a diehaving an extrusion opening 8 mm in diameter. The polyethylene was thenpressed with a cooled punch to make a crown having a low density liner.

(2) Crown A-2 (comparative example).

Crown shells formed from coated tin plate (A) were heated by highfrequency in the same manner as A-1 (except that the heating time wasfive seconds). Heating was done nearly uniformly until the crown shellbottom temperature reached 150° C. and then molten low densitypolyethylene was inserted as before to make the crown.

(3) Crown B-1 (comparative example).

Crowns were made by entirely the same method as for crown A-2 usingcrown shells formed from coated tin plate (B).

The following measurements and evaluations were made of the above threetypes of crowns.

Adhesive strength: Samples 5 mm wide and 27 mm long were cut out fromthe crown bottoms of each type of crown immediately after forming andafter standing for one week at room temperature. Measurements were madeof the peeling strength between the tin plte and the polyethylene. Themeasurements were done using tension, peeling at 180° at a rate of 20mm/min. and at a temperature of 20° C. The peeling strength was read ona chart as the values of four positions from the outer peripheries ofthe crowns to their centers. The results are shown in Table 1.

Environmental cracking: An aqueous solution of 0.01% RIPONOKKUSU NCI(made by Raion Yushi [Lion Fat & Oil Co., Ltd.]) was placed in glassbottles. The bottles were capped with the various types of crowns, letstand upside down in an atmosphere of 50° C. and were evaluated at thetime F20 until 20% had cracks formed (twelve out of sixty bottles). Thenumber of samples for each type at this time was sixty bottles.

Sealing: Commercial bottled beer (large bottles) were cooled to 4°-3° C.and opened, immediately capped with the test crowns and left on theirsides for one month at 37° C. to obtain their leakage rates.

In contrast to the fact that crowns A-1 of the present invention hadsatisfactory performance as container covers, comparative examples A-2and B-1 were not practical for use and had the following deficiencies.Crowns A-2 adhered over the entire surfaces, since the outer peripheriesof the lining material corresponding to the bottle mouths solidifiedwith the adhesive primer, these parts showed much more residual stressstrain during capping than the non-adhesive A-1 and B-1 crowns andenvironmental cracking occurred very easily. On the other hand, althoughcrown B-1 withstood environmental cracking, sealing was extremely poorcompared to crown A-1 of the present invention and the adhesive strengthof the liners decreased widely over passage of time. In contrast to thefact that crown A-1 had an adhesive strength in the liner that increasedgradually from the peripheral part to the central part, with crown B-1there was absolutely no distinction between the non-adhesive part andthe adhesive part. Because of this, stresses remained in the liner fromelongation from the center toward the periphery during liner forming,the overall adhesive strength of the liners in crown B-1 were not ableto withstand the force of restitution of this residual stress, and theadhesive strength was considered to be completely lost. For this reason,although crown B-1 comprised crowns with the same adhesive parts ascrown A-1, in practice sealing was surprisingly bad and they werecompletely unsuited for practical use.

                                      TABLE 1                                     __________________________________________________________________________                        A - 1                                                                         Example of                                                                          A - 2  B - 1                                                            the Present                                                                         Comparative                                                                          Comparative                                  FACTORS             Invention                                                                           Example                                                                              Example                                      __________________________________________________________________________    Peeling Strength                                                                        Distance                                                                           13                                                                              mm 0     1150   0                                            Immediately After                                                                       From 11                                                                              mm 250   1120   0                                            (kg/cm)   Center                                                                             8 mm 980   1140   1130                                                        4 mm 1140  1120   1130                                                        0 mm 1130  1120   1140                                         Peeling Strength                                                                        Distance                                                                           13                                                                              mm 0     960    0                                            After One Week                                                                          From 11                                                                              mm 0     940    0                                                      Center                                                                             8 mm 540   940    0                                                           4 mm 890   960    0                                                           0 mm 900   940    100                                          Environmental Cracking (F 20)                                                                     90 Days                                                                             5      90 Days                                      (Days)              or more      or more                                      Sealing (%)         0.1   35.8   95.8                                         (Leakage Rate)                                                                __________________________________________________________________________

EXAMPLE 2.

80 parts by weight of molecular weight about 3,000 bisphenol A typeepoxy resin, 20 parts by weight of amino resin (SUPABEKKAMIN P 138) and4 parts by weight of anhydride maleic acid modified polyethylene weredispersed or dissolved in organic solvent so that the solid fraction was28 wt % to make a primer. The primer was painted on aluminum sheet 0.2mm thick and dried by heating to make a coated aluminum sheet having acoating of 100 mg/dm² and then pictures and characters indicating a"win" or a "loss" for prize purposes were printed on the coating. Thisprinted coated sheet with the printed surface inside was pressformedinto cap shells (diameter 38 mm, height 17 mm) with a conventionallyknown method in a manner that the pictures and characters of theprinting were in the centers of the caps.

These cap shells were heated with high frequency by the method of FIG.3c with the heating apparatus of FIG. 4. At this time the high frequencyinduction heating coil with a distance of the conductors between centersof 10 mm, coil conductor diameter 6 mm, and coil conductor length 1,000mm was arranged so that the distance between the upper end of the coilconductors and the shell bottoms was 0.3 mm. Further, the temperaturedistribution was as shown by curve 2 in FIG. 8 from the results ofmeasurements of the shell bottom surface temperature by thermopaint. Theconductors were connected to a 400 kHz generator with an input to thehigh frequency generator of 15 kWh. Passage time of the shells throughthe heating apparatus was 1 second and the number of shell revolutionsat this time was about eight.

One gram particles of molten polyethylene were inserted into the heatedshells by the method of Example 1, and they were compressed with acooled punch to make caps having polyethylene liners. These caps wereused to seal glass bottles containing one liter of carbonated beverageand although stored for three months at room temperature, they showedabsolutely no problems in sealing.

Also, when the liners of the caps were given unsealing evaluations bytwenty women and children, they peeled very easily and the "wins" and"losses" could be distinguished. Further, in the same manner as inExample 1, the adhesive strength from the outer periphery to the centralpart was zero (19), 120 (10), 820 (5) and 815 (zero) kg/cm (where thevalues in parens are distances in mm from the center).

We claim:
 1. In a metal cap having a primer layer thereon covering theentire bottom surface thereof and a lining layer the underside of whichoverlies and completely contacts said primer layer and where the cap isadapted for sealing the open ends of containers; the improvementcomprising in that said primer layer has a thermal adhesiveness withrespect to said lining layer upon the heating of the cap and in thatonly a part of the lining layer at the bottom of the cap is thermallyadhesively affixed to said primer layer.
 2. In a metal cap according toclaim 1 having in addition an anti-corrosion layer positioned on saidcap between the surface of said cap and said primer layer.
 3. In a metalcap according to claim 1 wherein said lining layer comprises athermoplastic resin.